Cellular communication systems continue to grow in popularity and have become an integral part of both personal and business communications. Cellular telephones and similar devices allow users to place and receive phone calls almost anywhere they travel. Moreover, as the use of cellular telephone technology increases, so too has the functionality of cellular devices. For example, many cellular devices now incorporate Personal Digital Assistant (PDA) features such as calendars, address books, task lists, calculators, memo and writing programs, without limitation. These multi-function devices usually allow users to send and receive electronic mail messages wirelessly and access the internet via a long range wide area network (WAN), utilizing mobile network technology such as long term evolution (LTE) and WIMAX™, and/or via a short range wireless local area network (WLAN), for example, when the devices further include appropriate circuitry for WiFi™ and other IEEE 802.11™ standards. Each technology, when implemented, is provided with its own radio frequency (RF) transmission and reception frequencies, however the frequencies of the various technologies are often close together.
In a wireless communication device, if a plurality of communication technologies is supported, noise from one technology's radio can fall into the frequency band of another technology's radio, thus degrading operation. In further detail, if wideband noise from a WAN radio exhibits frequency components within the frequency band of the WLAN radio, significant degradation of the signal to noise ratio (SNR) in the receiver of the WLAN radio may occur during transmission by the WAN radio.
An example is illustrated in FIG. 1A. A communication device 10 comprising: a WAN transceiver 20; a first antenna 30; a power amplifier (PA) 40; a WLAN transceiver 50; a second antenna 60; and a low noise amplifier (LNA) 70. The output of WAN transceiver 20 is connected to the input of PA 40 and the output of PA 40 is connected to first antenna 30. WLAN transceiver 50 is connected to the output of LNA 70 and the input of LNA 70 is connected to second antenna 60. The wideband emissions emitted by PA 40 are radiated by first antenna 30, received by second antenna 60, amplified by LNA 70 and received as interference by WLAN transceiver 50. For simplicity only the transmit path of WAN transceiver 20 and the receive path of WLAN transceiver 50 is shown, however this is not meant to be limiting in any way. Additionally, WAN transceiver 20 may be provided with a plurality of receiving amplifier paths each connectable to one of a plurality of antennas so as to provide antenna diversity or multiple-input-multiple-output (MIMO), without exceeding the scope.
FIG. 1B illustrates the spectrum of the emissions of PA 40 with a 10 MHz bandwidth and operating with a power output of 23 dBm, where the x-axis represents frequency offset from the WAN center frequency and the y-axis represents emission power in dBm/MHz. In one embodiment, the center frequency of the WAN band is 2502 MHz and the uppermost channel of the WLAN frequency band is centered around 2462 MHz, i.e. only 40 MHz removed. As shown in curve 80, the emission power from PA 40 associated with WAN transceiver 20 is approximately −50 dBm/MHz at the WLAN center frequency. If, for example, the coupling factor of first antenna 30 to second antenna 60 is −15 dB, the resultant noise received by second antenna 60 at the center frequency of WLAN transceiver 50 is calculated as −50 dBm/MHz −15 dB=−65 dBm/MHz. When integrated over the 20 MHz passband of WLAN transceiver 50, the resultant noise level is −52 dBm. This will have a significant impact on the WLAN signal and will typically result in a 30-50 dB degradation in the sensitivity of WLAN transceiver 50, causing a significantly reduced range and throughput of the WLAN signal. Of course, this degradation only occurs when both the WAN and WLAN radios are operating simultaneously. However, this scenario can happen quite frequently, and is therefore a serious concern.
One potential solution to this problem is to place a filter 90 at the output of PA 40, as shown in communication device 10A of FIG. 1C, which is in all respects similar to communication device 10 with addition of filter 90, implemented as a very sharp bandpass filter, connected between the output of PA 40 and first antenna 30. Filter 90 passes the desired carrier bandwidth of WAN transceiver 20 while providing substantial attenuation of the power emissions in the carrier bandwidth of WLAN transceiver 50. The term substantial attenuation means an attenuation of at least 30 dB, and preferably at least 40 dB. Filter 90 may be implemented as either a surface acoustic wave (SAW) filter, a bulk acoustic wave (BAW) filter or any filter exhibiting a substantial attenuation of the frequencies in the carrier bandwidth of WLAN transceiver 50. Disadvantageously, these implementations of filter 90 exhibit significant insertion loss. For example, the ACPF-7025 available from Avago Technologies of San Jose, Calif., is a BAW filter designed to pass signals in the WAN carrier band from 2.5-2.7 GHz and attenuate signals in the WLAN carrier band by approximately 40 dB, thus providing an appropriate implementation for filter 90. Unfortunately, the ACPF-7025 exhibits a nominal insertion loss of 2.4 dB, with a worst case insertion loss of 5 dB over temperature and frequency. Between 42%-68% of the power transmitted from PA 40 is therefore absorbed as heat in filter 90. Thus, either the transmit power radiating from first antenna 30 will be reduced, resulting in shorter range, or the transmit power of WAN transceiver 20 must be dramatically increased, resulting in degraded battery life.
The above has been described in relation to simultaneous operation of a WAN transceiver and a WLAN transceiver, however this is not meant to be limiting in any way. The above description is similar for simultaneous operation of any two transceivers operating on frequencies that may interfere with each other.
What is desired, and not supplied by the prior art, is a system and method for simultaneous operation of two transceivers without significant SNR degradation of one of the signals and without a large overall insertion loss.